1. Field of the Invention
This invention generally relates to electronic power supplies, and more specifically to capacitive energy transfer DC-to-DC converters (DC/DC converters), such as charge pumps.
2. Related Art
DC/DC converter power supply circuits provide a DC output voltage based upon a DC source voltage. The output is typically at a different voltage than the input. As the term is used herein, DC/DC converters do not encompass voltage reduction regulator circuits that use a linear pass device, but rather involve energy transfer from input to output through an energy storage device, such as a capacitor or an inductor.
The DC/DC converters of interest herein are charge pumps, which obtain energy for the output voltage primarily by means of capacitive transfer from the source to the output. An inductor is not generally the primary energy transfer device in a charge pump, though of course hybrid devices are possible that employ inductive energy transfer in addition to capacitive energy transfer. A charge pump may derive an output voltage that is higher than a source voltage, or that is inverted from a source voltage, or that is referenced to a different voltage than the source voltage, and indeed may do all of these things concurrently.
Charge pumps may be implemented for a wide variety of purposes. They are well suited for integrated circuit fabrication because the devices and elements required are compatible with most integrated circuit fabrication techniques. For example, a charge pump may be employed to generate a negative gate bias supply for an integrated circuit that switches an antenna between send and receive circuitry of a transceiver, as shown in FIG. 1. Many wireless transceivers, such as cellular telephones, employ a single antenna for both receiving and transmitting. While such systems are receiving, an antenna 102 must be coupled to receive circuitry that may include, for example, a filter 104 and a low noise amplifier 106, to provide the received signal for further processing. However, while such systems are transmitting, the antenna 102 must be disconnected from the sensitive receive circuitry and coupled instead to relatively high power transmit circuitry. The transmit circuitry may include, for example, a power amplifier 108 and a transmit filter 110 to process a transmit signal.
A RF switch 112 may be used to perform such antenna switching functions. Ideally, such switches may be integrated together with the receive and/or transmit circuitry, and in any event are desirably very small, due to space limitations in portable transceivers such as mobile telephones and handy talkies. In order to achieve good performance from switching devices, such as MOSFETs, used to implement such RF switches, many designs need a special bias supply that extends negatively below the supply rails of the transmit and receive circuitry, such as a −3V supply. In view of the space and cost constraints of transceiver units such as mobile telephones, a charge pump is particularly suitable for generating such a bias supply, because it can be readily integrated into a very small circuit.
The RF switch 112 conveys relatively high power signals to the antenna 102 during transmission. However, during receive, the signal passed by the RF switch 112 may be measured in tens of nanovolts. Sharp noise transitions may have an extremely broad Fourier frequency content, and thus even signals at amplitudes on the order of millivolts may interfere unacceptably with reception if the signals have extremely fast edges. While the filter 104 can remove some noise, it is important that the RF switch 112 not introduce noise, particularly noise having components near the center frequency of the received signal. Thus, the receive/transmit switch of FIG. 1 illustrates one of many circumstances in which a charge pump may be desired for a circuit that nonetheless requires extremely low noise.
Unfortunately, noise generation is one of the most common drawbacks of charge pumps. Current spikes are typically coupled into both input and output supplies, together with voltage ripples and spikes. When a charge pump is integrated together with other devices, such electronic noise may be coupled throughout the circuitry of the integrated device by a variety of mechanisms that are difficult to control. Thus, a need exists for charge pumps that avoid generating excessive noise, so as to reduce charge pump noise injection into source supplies, output supplies, and related circuits.
The method and apparatus presented herein address this need for a low-noise charge pump. Various aspects of the method and apparatus described herein will be seen to provide further advantages, as well, for the design and construction of charge pumps that are relatively free of noise spurs.